In an image forming apparatus for forming an image on a recording material, in order to apply the proper heat depending on the kind of the used recording material in a fixing device (image heating device), it is preferable that the applied heat quantity is changed. Therefore, in a device disclosed in Japanese Laid-open Patent Application 2006-78555, when the kind of the recording material changes from plain paper to a thick sheet, which has a large thermal capacity, the image forming operation is started after the temperature rise of the fixing device up to a target temperature higher than a target temperature for the plain paper.
In a device disclosed in Japanese Laid-open Patent Application Hei 7-311506, when the kind of the recording material changes from plain paper to a thick sheet, the process speed (image forming speed, operation speed of the fixing device) is switched to an operation speed which is lower than for the plain paper, and then the image forming operation is started.
However, with the device disclosed in Japanese Laid-open Patent Application 2006-78555, the start of the image formation is delayed until a sufficient temperature rise of the fixing device, irrespective of the number of image formations on the thick sheets, and therefore, the productivity of image formation decreases. For example, in an apparatus capable of processing at a speed of 60 sheets per minute after the temperature rise, if the waiting time for the temperature rise is 30 sec, and only two sheets are processed, and the resulting productivity is only four sheets per minute.
In the apparatus disclosed in Japanese Laid-open Patent Application Hei 7-3111506, it is liable that the productivity of image formation decreases, because the image formation is started after switching of the process speed to a low speed after completion of the image formation on the plain paper, irrespective of the number of subsequent image formations on thick sheets. Here, the change of the process speed may require changes of a voltage application condition of image forming means and/or a heating condition of the fixing device, which may result in an even longer waiting time. For example, in the case that the image formation is carried out with a productivity of 60 sheets per minute after the switching of the process speed to that for thick sheets, and the condition change requires 30 second, the resultant productivity is only 4 sheets per minute if only two thick sheets are processed.